The present invention is directed to a method of reducing electrical transients in a relay coil and the structure of such a relay coil for providing the reduction in electrical transients. More particularly, the present invention is directed to the use of a flat or pancake shaped holding coil mounted in a position axially spaced from the operating coil and provided with a conductive shield in close proximity thereto to reduce electrical transients. The present invention is directed to the relay art, and particularly to the relay art for protective electrical relays. In other words, the present invention is particularly directed to applications wherein relays having armatures are utilized as protective relays to protect electrical circuits from overload, although the present invention may be utilized in relays for other applications. Particularly, the present invention is particularly adapted for use in direct current (DC) relays.
In a typical DC relay, an operating coil and a holding coil are provided. A relatively high value of operating current is required to close the armature of a DC relay. Only a small fraction of this value, sometimes referred to as a holding current, is required to maintain the armature in the operated position. If the operating current is maintained to hold the armature in the operated position, there may be considerable overheating and possible burnout of the operating coil. To prevent such adverse results, it has been customary to automatically switch a holding coil in series with the operating coil. The holding coil has a large number of turns of fine gauge wire which gives it a high resistance. The high resistance of the series connected coils reduces the holding current to a low value. However, the large number of turns of the holding coil produce sufficient ampere-turns to hold the armature in the operated position.
When these coils are deenergized, a substantial amount of energy is released. This transient energy may cause malfunctions and/or damage to other relays on the same circuit or to the relay itself.
ln the past, external devices, such as diodes, thyristors or capacitors, were used to supress these transients. Such components are subject to possible failure, which may even be caused by the momentary application of the wrong polarity of voltage across these devices. Even more serious and of even greater concern in the protective relaying field, and particularly as protective relays may be utilized in the protection of public utility electrical circuits, failures may not be evident until it is too late to make repairs, resulting in substantial damage to the electrical circuits of public utilities. External devices which require additional soldered joints in the relay to connect the external devices were always subject to the problems of possible cold solder joints or failure due to overheating. External components may also represent substantial additional cost in particular cases depending on the particular external device chosen to reduce the transients. The present invention eliminates electrical transients in relays in an efficient manner which obviates the need for external devices and eliminates their potential for detected and undetected failures. The present invention eliminates additional soldered connections and it reduces cost and is a reliable means of reducing transients in DC relays using a reliable method. The structure is built into the structure of the relay coil form and shield.